PROJECT SUMMARY Mantle cell lymphoma (MCL) is a rare but incurable subtype of B-cell lymphoma with an overall 5-year survival rate of approximately 50%. A multiple-center phase II study led by our center served as the basis for the U.S. Food and Drug Administration approval of ibrutinib to treat relapsed/refractory MCL. Our study revealed the unprecedented single-agent activity of ibrutinib in relapsed/refractory MCL, with an overall response rate of 68% and a complete response rate of 21%. Nevertheless, in our trial, we observed primary resistance to ibrutinib in approximately 43% of enrolled patients. Once patients relapse after ibrutinib treatment, the 1-year survival rate is only 22%; therefore, the vast majority of MCL patients who experience disease progression after ibrutinib treatment die within 12 months, demonstrating that standard-of-care approaches are failing and overcoming ibrutinib resistance remains an urgent unmet clinical need. To identify prevalent mutations underlying primary ibrutinib resistance, we performed mutational analysis on the clinical specimens of primary ibrutinib-resistant MCL patients and those who showed durable responses to this drug. The results showed that primary resistant tumors but not the tumors of the patients who displayed durable responses possess mutations predominantly in BCR/NF-kB pathway genes (39%), epigenetic modifier genes (24%), PIM1 and mTOR (18%), and oncogenes such as BCL2, MYC, ERBB4 (12%). Most of the mutant genes and signaling pathways identified are commonly deregulated in cancers and are known to play important roles in cell proliferation, metabolism, survival and malignant transformation; therefore, we deduce that genetic alterations in these pathways contribute to primary ibrutinib resistance and that combination treatments targeting these pathways will likely overcome primary ibrutinib resistance and enhance MCL treatment efficacies. However, the diversity in these genetic alterations and the possibility that multiple pathway aberrations may contribute to ibrutinib resistance create challenges in overcoming ibrutinib resistance with one or few treatment regimens, necessitating the personalization of care. In this proposal, we will use primary tumor cells and patient-derived xenografts from MCL patients who are primary resistant to ibrutinib to 1) identify the mechanisms that underlie ibrutinib resistance via molecular profiling, 2) simultaneously determine effective treatment combinations to overcome ibrutinib resistance in vitro, and 3) identify treatment combinations that successfully target ibrutinib resistance in vivo. The genetic mutations will be correlated with the in vitro treatment results to identify biomarkers capable of predicting individual treatment outcomes. Successful completion of the proposed study will identify effective novel combinatory treatments to overcome ibrutinib resistance, determine ibrutinib-resistant mechanisms and detect potential biomarkers capable of predicting individual patient treatment outcomes, ultimately establishing a practice-changing concept in the treatment of MCL patients, which will improve clinical outcomes through effective personalized therapy.